Now, techniques for stacking a plurality of recording layers are widely used to further increase the storage capacity of optical recording media. Such multilayer techniques have been studied for each of different optical recording media including read-only, recordable, and rewritable optical recording media. A proposed rewritable multilayer optical recording medium with a two-recording-layer structure includes a substrate and a first recording layer (L0 layer), a space layer, a second recording layer (L1 layer), and an optically transparent layer stacked in this order on the substrate. In this optical recording medium with the two-layer structure, a recording layer transparent to the laser beam used for recording on and reproduction from the first recording layer (hereinafter, for the sake of convenience, such a layer is referred to as a “semi-transparent recording layer”) is used as the second recording layer. The second recording layer is formed by stacking, for example, a dielectric material, a metal material, and a phase-change recording material to provide the ability to record and reproduce information. Typically, the second recording layer has a structure in which a first dielectric layer, a reflective metal layer, a second dielectric layer, a phase-change recording layer, and a third dielectric layer are stacked in this order on a space layer (see, for example, Patent Document 1).
A product having a three-recording-layer structure for larger storage capacity is also commercialized, in which a first recording layer (L0 layer), a space layer, a second recording layer (L1 layer), a space layer, a third recording layer (L2 layer), and an optically transparent layer are stacked in this order on a substrate. There are also proposed methods for increasing the recording density, which include reducing the shortest mark length to increase the density in the line direction and recording information on both lands and grooves.
In the recording medium with such a two- or three-layer structure, the shallower layer needs to have higher transmittance so that information can be recorded on and reproduced from the layer most distant from the laser beam side. In that case, the recording layers with high absorption coefficient and the reflective layer need to be relatively thin. Particularly in a case where the recording layers are made thin, the crystallization speed can decrease so that the recording speed for rewriting can decrease and the speed of recording on the phase-change recording medium can become significantly lower than that on recordable media.
On the other hand, a technique for improving the speed of recording on a phase-change recording medium includes optimizing the material for the phase-change recording layer to increase the crystallization speed. For example, GeSbSn and InSb (see, for example, Patent Documents 2 and 3), and GaSbGe (see, for example, Patent Document 4) are proposed as the phase-change recording material for such a purpose.